1. Field of the Invention
This invention relates generally to holograms and hologram recording techniques, and more particularly has reference to reducing the noise and improving the angular sensitivity of a transmission type holographic optical element.
2. Description of the Prior Art
In modern aircraft, both military and commercial, it is important that certain information be clearly and unobtrusively presented to the pilot while he is viewing the outside world. The head-up display (HUD) has been developed for this purpose. It provides a display of alphanumerics, symbology, gun sight reticle (for fighter aircraft) and other information superimposed on the pilot's forward field of view.
In a conventional HUD, the information display is generated on a high-brightness cathode ray tube and then projected through a relay lens system onto a transparent combiner screen. Typically, the combiner is a partially silvered mirror, located between the pilot and the aircraft windscreen. The combiner directs the projected images to the pilot's eyes, while at the same time affording him an unobstructed view of the outside world as seen through the transparent combiner and the windscreen.
More recently, the combiner for a head-up display has included a holographic film. Such a film consists of one or more layers of photoreactive gelatin that have been exposed by a holographic process so as to record a directionally sensitive, reflective-type volume holographic fringe pattern. The holographic film has better reflection and see-through capability than a partially silvered mirror.
There are certain disadvantages, however, to using a reflection hologram in a HUD combiner. Significantly, a reflection hologram combiner often must be positioned very close to the windscreen and very far from the pilot's eyes to achieve the desired field of view. This not only limits the freedom of design choices, but also results in design trade-offs in terms of field of view, since the available space to accommodate a combiner decreases as one moves closer to the aircraft windscreen.
One of the advantages of using a transmission hologram in the optical design would be the ability to move the combiner further from the windscreen and closer to the pilot's eyes. In this location, a combiner of a given size would represent a wider field of view. The results would be increased latitude of HUD design and the ability to produce a HUD with a larger field of view than in comparable instrument with a reflection hologram combiner.
Unfortunately, transmission holograms have not proved successful for use in a head-up display because of the noise produced by diffracted sunlight. When a bright source of diffuse light such as the direct rays of the sun strike a transmission hologram, unwanted light (noise) of high intensity is diffracted and transmitted through the hologram in all direction, reducing image contrast and obscuring the desired informational display.
It would be highly desirable to overcome this problem and remove the restriction on the use of transmission holograms in a head-up display.